1. Field of the Invention
The present invention relates to a plasma processing system that produces a plasma by the energy of radio-frequency waves, such as microwaves and processes a substrate, such as a semiconductor wafer, by using the plasma.
2. Description of the Related Art
A semiconductor device fabricating method includes a plasma process that processes a semiconductor wafer (hereinafter referred to simply as xe2x80x9cwaferxe2x80x9d) with a plasma. FIG. 22 shows a known microwave plasma processing system capable of carrying out such a plasma process. This known plasma processing system has a vacuum vessel 9 internally provided with a table 90 for a wafer W, a microwave transmitting window 91 of, for example, quartz forming the top wall of the vacuum vessel 9, a slot antenna 92 disposed above the microwave transmitting window 91, and an electromagnetic shielding member 96 of a cylindrical shape disposed above the microwave transmitting window 91 and joined to the upper end of the vacuum vessel 9. A waveguide 94 guides microwaves generated by a microwave power supply system 93 to the antenna 92 to propagate the microwaves in the vacuum vessel 9. A plasma is produced by ionizing a process gas supplied into the vacuum vessel 9 by a gas supply unit 95 by the microwaves. The plasma is used for a film forming process for depositing a film on the surface of the wafer W or for an etching process.
A uniform plasma must be produced in this plasma processing system to process the surface of the wafer W highly uniformly. The field strength distribution of the microwaves is one of the factors that dominate the uniformity of the plasma. It is mentioned in a Japanese patent laid-open publication JP-A No. Hei 3-68771 that the radiant intensity distribution (field strength distribution) of microwaves can be optionally changed by properly designing the construction of the antenna, microwaves are emitted according to the strength of a standing wave produced right in front of the antenna and hence a radiant intensity distribution is uniform when a microwave absorber is disposed immediately in front of the antenna (the exit end of a microwave transmission path) to suppress the standing wave.
The inventors of the present invention varied the mode of microwave radiation by attaching a metallic tape to the antenna 92 and observed the plasma through a CCD camera mounted on the table 90. It was found that brightness distribution in the plasma varies scarcely. It is known from this fact that, even though the field strength distribution of the microwaves can be adjusted by the antenna 92, there exists a factor that disturbs the field strength distribution of the microwaves in a space between the antenna 92 and a plasma luminescent area. The inventors of the present invention acquired a knowledge that a standing wave is produced in a space between the antenna 92 and a cease area, i.e., a nonluminous region between the microwave transmitting window 91 and a luminous plasma region. It is thought that the standing wave is a transverse wave that is generated by the reflection of electromagnetic waves from a side wall when a microwave propagation space is large. Therefore, the uniformity of the field strength distribution of the microwaves is deteriorated by the standing wave and a plasma of irregular density is produced, which makes the highly uniform processing of a surface difficult.
The present invention has been made under such circumstances and it is therefore an object of the present invention to provide a plasma processing system capable of suppressing the generation of a standing wave in a space between an antenna and a plasma luminescent area, of producing a highly uniform plasma and of achieving highly uniform processing.
The present invention provides a plasma processing system that propagates plasma-producing radio-frequency waves generated by a radio-frequency power supply system through a flat antenna and a radio-frequency wave transmitting window into a vacuum vessel, produces a plasma by ionizing a process gas supplied into the vacuum vessel by the energy of the radio-frequency waves and processes a substrate placed on a substrate table arranged in the vacuum vessel with the plasma, characterized in that the system is configured so that the system is capable of suppressing a standing wave.
The present invention is featured by an electromagnetic wave absorber disposed so as to surround a region between a surface of the radio-frequency wave transmitting window on the side of a vacuum atmosphere in the vacuum vessel and the antenna. Preferably, the electromagnetic wave absorber is divided into a plurality of divisions, the divisions are arranged at circumferential intervals with spaces formed between the adjacent divisions, and the circumferential length of the divisions and the circumferential length of the spaces between the divisions are smaller than xcexg/2, where xcexg is the wavelength of the radio-frequency waves at that part.
It is another features of the present invention that a region between a region between the radio-frequency wave transmitting window and the plasma luminescent area, and a surface of the radio-frequency wave transmitting window on the side of the antenna is divided in a direction perpendicular to the direction of propagation of the radio-frequency waves by conductive members. Preferably, one end part of a length in the range of 5 to 10 mm of each conductive member on the side of the table extends in the plasma luminescent area.
Preferably, the conductive members include a circular or annular first conductive element, and an annular second conductive element surrounding the first conductive element and concentric with the first conductive element. Preferably, the radial distance R2 between the concentric first and the second conductive elements meet an inequality: xcex/2xe2x89xa6R2 less than xcex, where xcex is the wavelength of the radio-frequency waves. Preferably, the inside diameter R1 of the first conductive element meets an inequality: xcex/2xe2x89xa6R1 less than xcex. The region may be divided by a plurality of radial conductive elements arranged at angular intervals. A region in which the conductive member is disposed may be defined by only the radio-frequency wave transmitting window. In such a case, the radio-frequency wave transmitting window is divided by the conductive member. The region may be, for example, a region between the radio-frequency wave transmitting window and the plasma luminescent area. In such a case, the conductive member may be used also as a gas supply unit for supplying a process gas to a substrate placed on the table.